The invention relates to a method of determining an NMR distribution in a region of a body which is situated in a steady, uniform magnetic field, including the steps of:
(a) generating an r.f. electromagnetic pulse in order to cause a precessional motion of the magnetization of the nuclei in the body, thus generating a resonance signal,
(b) then generating, after a preparation period, a steady gradient magnetic field and an alternating, periodic gradient magnetic field during a measurement period of several measurement periods, said measurement period (periods) being divided into a number of sampling intervals for taking a number of signal samples of the resonance signal,
(c) then repeating, each time after a waiting period, the steps (a) and (b) a number of times, the duration of the preparation period and/or the integral over the preparation period of at least one gradient magnetic field applied during the preparation period each time having a different value in order to obtain a group of signal samples from which, after signal transformation thereof, an image of a nuclear magnetization is determined.
The invention also relates to a device for determining an NMR distribution in a region of a body, comprising:
(a) means for generating a steady, uniform magnetic field,
(b) means for generating r.f. electromagnetic radiation,
(c) means for generating a steady gradient magnetic field,
(d) means for generating an alternating, periodic gradient magnetic field,
(e) sampling means for taking signal samples of a resonance signal generated by the means specified in the paragraphs (a) and (b) in the presence of a steady gradient magnetic field and of an alternating gradient magnetic field generated by the means specified in paragraphs (c) and (d),
(f) processing means for processing of the signal samples in order to obtain an NMR distribution, and
(g) control means for controlling at least the means specified in the paragraphs (b) to (f) for generating, conditioning, and sampling a number of resonance signals and for processing the signal samples.
Such a method and device are known from Netherlands patent application NL-A No. 82.03519 corresponding to U.S. Pat. No. 4,527,124. According to the known method, a periodic alternating gradient magnetic field is generated during the measurement period, the period of said gradient field being equal to the sampling interval, at least one additional signal sample being taken in each sampling interval.
As explained in said Netherlands patent application NL-A No. 82.03519, the use of the alternating gradient magnetic field and the taking of the additional signal samples ensure that at least two rows of a (two-dimensional) image frequency matrix will have been filled after the sampling of a resonance signal (FID or spin echo signal). Thus, the duration of a measurement cycle is reduced to one half (one third, one quarter) when one (two, three) additional signal samples are taken, respectively. Because the duration of a resonance signal amounts to only some tens of milliseconds, the taking of 128 or 256 signal samples (in a row in the image frequency matrix) will require a sampling interval in the order of magnitude of 100 .mu.s, which means that the frequency of the additional gradient magnetic field must amount to 10 kHz. This comparatively high frequency of the alternating gradient magnetic field limits the maximum number of rows of the image frequency matrix which can be filled by the sampling of a single resonance signal. The maximum distance .DELTA.k between two rows filled by the sampling of a resonance signal amounts to: ##EQU1## in which 1/2t.sub.m is the first half period of the periodic, alternating gradient magnetic field, .gamma. is the gyromagnetic ratio, and G(.tau.) is the alternating gradient magnetic field. The maximum distance .DELTA.k determines the maximum number of rows in the image frequency matrix filled after the sampling of a resonance signal and is proportional to the amplitude of the applied alternating gradient magnetic field. The amplitude of the alternating gradient magnetic field cannot be increased at random, because the rate of change dG/dt of the alternating gradient magnetic field must remain within health safety limits imposed. This rate of change dG/dt is proportional to the product of the amplitude and the frequency of the alternating gradient magnetic field. Because the frequency (10 kHz) is comparatively high, a maximum permissible amplitude will be quickly reached. If the period of time required for collecting all signal samples were to be reduced to one quarter, the amplitude of the alternating field would have to be increased by a factor 4.